Microbial communities in top- and subsoil of repacked soil columns respond differently to amendments but their diversity is negatively correlated with plant productivity.

Organic and inorganic amendments with equivalent nutrient content may have comparable fertilizer effects on crop yield, but their effects on the soil microbial community and subsequent plant-soil-microbe interactions in this context are unknown. This experiment aimed to understand the relationship between soil microbial communities, soil physicochemical characteristics and crop performance after addition of amendments to soil. Poultry litter and synthetic fertilizer with balanced total nitrogen (N) content equivalent to 1,200 kg ha-1 were added to the topsoil (0-10 cm) or subsoil layer (20-30 cm) of repacked soil columns. Wheat plants were grown until maturity. Soil samples were taken at Zadoks 87-91 (76 days after sowing) for analysis of bacterial and fungal communities using 16S and ITS amplicon sequencing. The interaction between amendment type and placement depth had significant effects on bacterial and fungal community structure and diversity in the two soil layers. Addition of poultry litter and fertilizer stimulated or suppressed different taxa in the topsoil and subsoil leading to divergence of these layers from the untreated control. Both amendments reduced microbial community richness, diversity and evenness in the topsoil and subsoil compared to the nil-amendment control, with these reductions in diversity being consistently negatively correlated with plant biomass (root and shoot weight, root length, grain weight) and soil fertility (soil NH4+, shoot N). These results indicate that in this experimental system, the soil microbial diversity was correlated negatively with plant productivity.